The invention relates to an electrostatic chuck (ESC) useful for processing substrates such as semiconductor wafers. The ESC can be used to support a semiconductor substrate in a plasma reaction chamber wherein etching or deposition processes are carried out. The ESC is especially useful for high temperature plasma etching of materials such as platinum which are not volatile at low temperatures.
Vacuum processing chambers are generally used for etching and chemical vapor depositing (CVD) of materials on substrates by supplying an etching or deposition gas to the vacuum chamber and application of an RF field to the gas to energize the gas into a plasma state. Examples of parallel plate, transformer coupled plasma (TCP) which is also called inductively coupled plasma (ICP), and electron-cyclotron resonance (ECR) reactors are disclosed in commonly owned U.S. Pat. Nos. 4,340,462; 4,948,458; and 5,200,232. Vacuum processing chambers are typically designed to meet performance specifications which depend on the process to be carried out therein. Thus, the particular plasma generating source, vacuum pumping arrangement and substrate support associated with the particular processing chamber must be customized or specially designed to meet the performance specifications.
Substrates are typically held in place within the vacuum chamber during processing by substrate holders. Conventional substrate holders include mechanical clamps and electrostatic clamps (ESC). Examples of mechanical clamps and ESC substrate holders are provided in commonly owned U.S. Pat. Nos. 5,262,029, 5,880,922 and 5,671,116. Substrate holders in the form of an electrode can supply radiofrequency (RF) power into the chamber, as disclosed in U.S. Pat. No. 4,579,618. Electrostatic chucks of the monopolar type utilize a single electrode. For instance, see U.S. Pat. No. 4,665,463. Electrostatic chucks of the bipolar type utilize mutual attraction between two electrically charged electrodes which are separated by a dielectric layer. For instance, see U.S. Pat. Nos. 4,692,836 and 5,055,964.
Substrates including flat panel displays and smaller substrates can be cooled by the substrate holder during certain processing steps. Such cooling is performed by the application of a gas, such as helium, between the substrate holder and the opposed surface of the substrate. For instance, see U.S. Pat. Nos. 5,160,152; 5,238,499; 5,350,479; and 5,534,816. The cooling gas is typically supplied to channels or a pattern of grooves in the substrate holder and applies a back pressure to the substrate.
Substrate supports for vacuum processing chambers are typically mounted on a bottom wall of the chamber making servicing and replacement of the substrate support difficult and time consuming. Examples of such bottom mounted substrate supports can be found in U.S. Pat. Nos. 4,340,462; 4,534,816; 4,579,618; 4,615,755; 4,948,458; 5,200,232; and 5,262,029. A cantilevered support arrangement is described in commonly owned U.S. Pat. Nos. 5,820,723 and 5,948,704.
High temperature electrostatic chucks incorporating clamping electrodes and heater elements have been proposed for use in chemical deposition chambers. See, for example, U.S. Pat. Nos. 5,730,803; 5,867,359; 5,908,334; and 5,968,273 and European Patent Publication 628644 A2. Of these, EP""644 discloses an aluminum nitride chuck body having an RF metallic electrode plate which is perforated with holes to form a mesh and a heater embedded therein, the chuck body being supported on an alumina cylinder such that the outer periphery of the chuck body extends beyond the cylinder. The ""803 patent discloses a chuck body of silicon nitride or alumina having an electrical grid of Mo, W, Wxe2x80x94Mo and a Mo heater coil wire embedded therein, the chuck body being supported by a Mo heat choke cylinder which surrounds a Cu or Al water cooled cooling plate in thermal contact with the chuck body by a thermal grease which allows differential expansion between the chuck body and the cooling plate. The ""359 patent describes a chuck operational at temperatures on the order of 500xc2x0 C., the chuck including sapphire (single crystal Al2O3) layers brazed to opposite sides of a niobium electrode and that assembly brazed to a metal base plate. The ""334 patent describes a chuck for use at temperatures in excess of 175xc2x0 C., the chuck including polyimide films on either side of a monopolar or bipolar electrode with the lower polyimide film self adhered to a stainless steel platen. The ""273 patent discloses a layered chuck body including an aluminum nitride top layer, an electrode, an aluminum nitride layer, a metal plate, a heater, a metal plate and an aluminum composite, the chuck body being supported by a cylinder such that the outer periphery of the chuck body extends beyond the cylinder.
Some ESC designs use a heat conduction gas such as helium to enhance conduction of heat between adjacent surfaces of the wafer support. For instance, U.S. Pat. No. 5,155,652 describes an ESC having layers including an upper pyrolytic boron nitride layer or optionally polyimide, alumina, quartz, or diamond, an electrostatic pattern layer comprised of a boron nitride substrate and a conductive pattern of pyrolytic graphite thereon, a heater layer comprised of a boron nitride substrate and a conductive pattern of pyrolytic graphite thereon, and a heat sink base of KOVAR (NiCoFe alloy with 29% Ni, 17% Co and 55% Fe). The heat sink base includes water cooling channels in a lower portion thereof and chambers in an upper surface thereof which can be maintained under vacuum during heatup of the chuck or filled with helium to aid in cooling of a wafer supported by the chuck. U.S. Pat. No. 5,221,403 describes a support table comprised of an upper member which supports a wafer and a lower member which includes a liquid passage for temperature control of the wafer, the upper member including an ESC constituted by a copper electrode between polyimide sheets and a gap between contacting surfaces of the upper and lower members being supplied a heat conduction gas.
Commonly owned U.S. Pat. No. 5,835,334 describes a high temperature chuck wherein helium is introduced between contacting surfaces of a lower aluminum electrode and an electrode cap which is bolted to the lower electrode, the electrode cap comprising anodized aluminum or diamond coated molybdenum. A protective alumina ring and O-ring seals minimize leakage of the coolant gas between the electrode cap and the lower electrode. The electrode cap includes liquid coolant channels for circulating a coolant such as ethylene glycol, silicon oil, fluorinert or a water/glycol mixture and the lower electrode includes a heater for heating the chuck to temperatures of about 100 to 350xc2x0 C. To prevent cracking of the anodization due to differential thermal expansion, the electrode cap is maintained at temperatures no greater than 200xc2x0 C. In the case of the diamond coated molydenum electrode cap, the chuck can be used at higher temperatures.
International Publication WO 99/36956 describes a process for plasma etching a platinum electrode layer wherein a substrate is heated to above 150xc2x0 C. and the Pt layer is etched by a high density inductivity coupled plasma of an etchant gas comprising chlorine, argon and optionally BCl3, HBr or mixture thereof. U.S. Pat. No. 5,930,639 also describes a platinum etch process wherein the Pt forms an electrode of a high dielectric constant capacitor, the Pt being etched with an oxygen plasma.
Although there have been some attempts to provide improved chuck designs for use at high temperatures, the high temperatures impose differential thermal stresses which are detrimental to use of materials of different thermal expansion coefficients. This is particularly problematic in maintaining a hermetic seal between ceramic materials such as aluminum nitride and metallic materials such as stainless steel or aluminum. As such, there is a need in the art for improved chuck designs which can accommodate the thermal cycling demands placed upon high temperature chuck materials. In particular, as larger work pieces are used to increase productivity and for larger display devices a design which can accommodate greater thermal expansion is needed.
The invention provides a substrate support useful in a high temperature vacuum processing chamber. In a preferred embodiment, the substrate support is an electrostatic chuck comprising a chuck body, a heat transfer body and an expansion assembly which can form an outer tubulation therebetween. The chuck body comprises an electrostatic clamping electrode and optional heater element, the electrode being adapted to electrostatically clamp a substrate such as a semiconductor wafer on an outer surface of the chuck body. The heat transfer body is separated from the chuck body by a plenum located between spaced-apart surfaces of the chuck body and the heat transfer body. The heat transfer body is adapted to remove heat from the chuck body by heat conductance through a heat transfer gas in the plenum.
The invention also provides an expansion assembly for the substrate support which accommodates differential thermal expansion of the chuck body and the heat transfer body. The expansion assembly attaches an outer periphery of the chuck body to the heat transfer body. According to a preferred embodiment, the expansion assembly forms an outer tubulation which, in combination with the chuck body, an inner tubular section, and the heat transfer body, encloses the plenum. A vacuum seal between the chuck body and the expansion assembly maintains a hermetic seal of the plenum during thermal cycling of the chuck body. Preferably the seal is a compressible metal seal, most preferably a C-ring seal.
According to a preferred embodiment, the heat transfer body comprises a cooling plate having at least one coolant passage therein in which coolant can be circulated to maintain the chuck body at a desired temperature and the plenum is an annular space extending over at least 50% of the underside of the chuck body. In this embodiment, the heat transfer body includes a gas supply passage through which heat transfer gas flows into the annular space. According to a preferred embodiment, the chuck body includes gas passages extending between the plenum and the outer surface of the chuck body. The gas passages can be provided in any suitable arrangement. For instance, if the outer portion of the chuck body tends to become hotter than the central portion thereof, the gas passages can be located adjacent to the expansion assembly so that the heat transfer gas flows from the plenum to the underside of an outer periphery of the substrate during processing thereof. In a preferred embodiment, the substrate can be heated controllably to above about 80xc2x0 C., more preferably above about 200xc2x0 C. up to about 350xc2x0 C. or higher. In a preferred embodiment, heat conduction between the chuck body and the heat transfer body through the expansion assembly is limited by a thin annular section functioning as an expansion joint and heat choke so that heat transfer is modulated by controlling the pressure of heat transfer gas in the plenum. In a more preferred embodiment, direct heat conduction from a high temperature zone which comprises the chuck body and a low temperature zone comprising the heat transfer body (i.e., a cooling plate) through the expansion assembly heat choke is limited such that the temperature difference between the outer perifery of the chuck body and an inner portion of the chuck body is less than about 4 to 5xc2x0 C., most preferably less than about 2xc2x0 C.
According to the preferred embodiment, the chuck body comprises a metallic material such as aluminum or alloy thereof or a ceramic material such as aluminum nitride. Lift pins can be used to raise and lower a substrate. For instance, the heat transfer body can include lift pins such as cable actuated lift pins mounted thereon, the lift pins being movable towards and away from the chuck body such that the lift pins travel through holes in the chuck body to raise and lower a substrate onto and off of the chuck body.
The expansion assembly can include a lower mounting flange adapted to attach to the heat transfer body and an expansion joint such as a flexible metal part which can also function as a heat choke. The expansion joint can comprise an inner annular section connected to the lower flange by a curved section and welded or brazed to an upper mounting flange or vice versa. The chuck body may be attached to the upper mounting flange of the expansion assembly by a clamping arrangement such as a continuous clamping ring or a sectioned ring. A hermetic seal is provided between the chuck body and the upper mounting flange by a seal member such as a metal C-ring seal compressed between the outer peripheral lower surface of the chuck body and the top surface of the upper mounting flange of the expansion assembly. The clamping ring may be made of material chosen with a thermal expansion coefficient matched to that of the chuck body to minimize strain of the clamping ring during thermal cycling of the chuck body.
The chuck body can include a ceramic or metallic tubular section extending from a central portion of the underside of the chuck body such that an outer surface of the tubular section defines a wall of the plenum, the tubular section being supported in floating contact with the heat transfer body with a hermetic seal therebetween. The interior of the tubular section can include power supplies supplying RF and DC power to the clamping electrode and AC power to the heater element, and/or a temperature measuring arrangement for monitoring temperature of the chuck body.
According to an embodiment of the invention, the chuck is a replaceable electrostatic chuck for a vacuum processing chamber wherein the chuck includes a chuck body attachable to an expansion assembly. The chuck comprises an electrode having an electrical contact attachable to an electrical power supply which energizes the electrode sufficiently to electrostatically clamp a substrate on an outer surface of the chuck body. According to an embodiment of the invention, the electrostatic chuck comprises a conveniently replaceable expansion assembly and a conveniently replaceable chuck body so that the useful life of the electrostatic chuck may be extended through economical servicing. The invention therefore also provides an expansion assembly adapted to be removably attached between a chuck body and a heat transfer body of the present invention.
The invention also provides a method of processing a substrate in a vacuum process chamber wherein the substrate is electrostatically clamped on a chuck body. The method comprises clamping a substrate on an outer surface of the electrostatic chuck of the present invention by energizing the electrode, heating the substrate with a heating element in the chuck body, supplying a heat transfer gas to the plenum, the gas passing through holes in the chuck body to a gap between an underside of the substrate and the outer surface of the chuck body, controlling the temperature of chuck body and the substrate by heat conductance through the heat transfer gas supplied to the plenum, and processing the substrate.
According to a preferred embodiment, the method further comprises supplying process gas to the chamber and energizing the process gas into a plasma and etching an exposed surface of the substrate with the plasma during the processing step. However, an exposed surface of the substrate can be coated during the processing step. The process gas can be energized into the plasma by any suitable technique such as supplying radiofrequency energy to an antenna which inductively couples the radiofrequency energy into the chamber. During the processing step, the substrate can be heated by supplying power to a heater element embedded in the chuck body. Prior to clamping the substrate, the substrate can be lowered onto the outer surface of the chuck body with lift pins mounted on the heat transfer body, the lift pins passing through openings in an outer portion of the chuck body. In order to withdraw heat from the chuck body, the method can include circulating a liquid coolant in the heat transfer body. Temperature changes in the substrate can be monitored with a temperature sensor supported by the heat transfer body and extending through a hole in the chuck body. In the case of plasma etching a layer of platinum during the processing step, the substrate can be heated to a temperature of over 200xc2x0 C.
According to the method, it is possible to achieve a desired heat distribution across the chuck body by removing heat from the chuck body through multiple heat paths. Further, it is possible to adjust the amount of heat removed through these heat paths by changing the pressure of the heat transfer gas in the plenum. For instance, since the ceramic or metallic tubular extension at a central portion of the underside of the chuck body conducts heat from the chuck body to the heat transfer body, the method can include adjusting pressure of the heat transfer gas in the plenum so that heat removed through a first heat path provided by the heat transfer gas in the plenum balances heat removed through a second heat path provided by an outer tubulation and heat removed through a third heat path provided by the central tubular extension.